A diode-laser (edge-emitting semiconductor laser) bar usually includes a plurality of individual diode-lasers (emitters) distributed along a “bar” comprising a plurality of semiconductor layers epitaxially grown on an electrically conductive semiconductor substrate. Such a bar usually has a length of about 10 millimeters (mm), a width of between about 1 mm and 1.5 mm, and a thickness of between about 100 micrometers (μm) and 300 μm. The emitters (diode-lasers) of the bar are formed in the epitaxial layers.
In a diode-laser bar configured to deliver near infrared radiation with a power of about 1 Watt (W) per emitter or more, the width of the emitters is typically between about 50 μm and 200 μm. Usually, the wider the emitter the higher the power output of an individual emitter. The number of emitters in a bar is determined by the length of the bar, the width of the emitters, and the spacing therebetween. Twenty emitters per bar is not an uncommon number of emitters per bar.
The emitters are aligned in the bar along an axis generally designated the slow-axis of the bar. This axis is so named because the beam emitted by an emitter has a relatively low divergence in this axis, for example about 10°. An axis perpendicular to the slow axis is designated the fast axis, as in this axis the emitted beams have a divergence of about 35° or even greater. In most applications of a diode laser bar it is necessary to collimate the emitted beams in the fast-axis. In a diode-laser bar package this is typically done by aligning a positive cylindrical lens, having a length about equal to the length of the bar, with the slow axis of the emitters at about a focal length of the lens, usually less than 1 millimeter (mm) from the emitters. A diode-laser bar package usually includes a heat-sink to which the bar is thermally connected, the fast-axis collimating lens fixed to the package in some way, and electrical arrangements for connecting electrical current to the emitters of the diode-laser bar. All of these components are assembled with an assortment of clamps, solders, and adhesives.
It is essentially impossible to have all of these components, clamps, solders, adhesives matched for thermal expansion coefficient. This is particularly true of the cylindrical lens and mounting arrangements thereof. As a result of this, the cylindrical lens in most commercial diode-laser bar packages is very vulnerable to misalignment due to thermal cycling. A few micrometers misalignment of the cylindrical lens in the fast axis can cause problematic changes in beam pointing. Most commercially available diode-laser bar packages will experience fatal lens misalignment in less than 50 thermal cycles between −55° C. and 85° C. Improving thermal-cycle lifetime of the cylindrical lens mounting in diode-laser bar packages presents a continuing challenge to manufacturers of such packages.